fsck_ufs: fix segfault with gjournal

The segfault was being hit in closedisk() while trying to free a
non-pointer.

The fix for this bug differs between stable/12 and stable/13.

PR: 245907
Submitted by: longwitz@incore.de
Differential Revision: https://reviews.freebsd.org/D30537

(cherry picked from commit 441e69e419effac0225a45f4cdb948280b8ce5ab)